The present invention relates to a process for forming films of silica (silicon dioxide) on substrates, and more particularly an improvement of a process for forming silica films wherein silica films are deposited on substrates by bringing the substrates into contact with an aqueous solution of hydrosilicofluoric acid (hexafluorosilicic acid) supersaturated with silica.
There is known a liquid phase deposition (LPD) process wherein silica films are deposited on the surface of a substrate by immersing the substrate into an aqueous solution of hydrosilicofluoric acid supersaturated with silica, as disclosed in U.S. Pat. No. 2,505,629 and Japanese Patent Publications Kokai No. 57-196744, No. 61-281047, (U.S. Pat. application Ser. No. 07/580,356) and No. 62-20876. The supersaturation state of the treating solution is achieved, for instance, by adding a reagent capable of accelerating the decomposition of H.sub.2 SiF.sub.6, e.g. boric acid, to an aqueous solution of H.sub.2 SiF.sub.6 substantially saturated with silica, or by elevating the temperature of an aqueous solution of H.sub.2 SiF.sub.6 substantially saturated with silica.
In the LPD process, it is effective for increasing the rate of deposition of silica to increase the feed rate of the reagent or a heat energy to accelerate the decomposition of H.sub.2 SiF.sub.6. However, it is known that if the feed rate is too fast, silica particles are produced in the treating solution and the deposition rate is rather decreased. This means that the deposition efficiency, namely efficiency of film formation, as represented by a ratio of the amount of silica deposited to the amount of reagent or heat energy fed, is decreased.
In order to increase the deposition rate with controlling the production and growth of silica particles, namely in order to increase the deposition efficiency, there is proposed a process wherein the reagent or heat energy is continuously fed, and silica particles produced in the treating solution are filtered out while circulating a part of the treating solution, as disclosed in Japanese Patent Publications Kokai No. 60-33233 and No. 63-102738.
Also, in order to form a dense silica film in the LPD process, it is proposed to conduct the silica deposition at a high temperature of the treating solution (e.g. Japanese Patent Publication Kokai No. 1-28377), or it is proposed to use an aqueous H.sub.2 SiF.sub.6 solution having a high concentration as the treating solution (e.g. Japanese Patent Publication Kokai No. 1-17871).
However, in these proposed processes, Si components presumed to be H.sub.2 SiF.sub.6 or SiF.sub.4 evaporate vigorously from the treating solution, so silica particles are formed at the interface between the treating solution and the atmosphere and on the wall of a treating solution tank at positions over the surface of the treating solution. The formation of silica particles is a loss of a component capable of forming a silica film in the treating solution. The evaporation of effective component results in decrease of deposition efficiency and moreover pollutes the working environment.
In order to prevent the evaporation of Si components from the treating solution, it is proposed to eliminate the interface of the treating solution and the surrounding gas by floating a liquid material having no or low reactivity with the treating solution and a lower specific gravity than the treating solution (e.g. Japanese Patent Publication Kokai No. 2-289414). However, the representative liquid materials used to form a barrier layer on the treating solution are aliphatic hydrocarbons of the formula: C.sub.n H.sub.2n+2 wherein n is an integer of 4 to 31, and the industrial use thereof is dangerous because in general they have an inflammability and a high vapor pressure or are harmful. Further, when the temperature of treating solution is raised in order to form a dense silica film, the floating liquid material, in its turn Si components, are evaporated from the treating solution.
It is an object of the present invention to provide a process for forming silica films on substrates in a high efficiency.
A further object of the present invention is to provide a process for forming silica films at a high silica deposition rate without loss of Si components effective for the film formation resulting from formation of silica particles in the treating solution and evaporation of Si components from the treating solution.
These and other objects of the present invention will become apparent from the description hereinafter.